A major aim of this grant is to investigate the developmental origin of the skeleton-forming cells in the head. An understanding of how these skeletal cells form in the embryo will aid in our long-term goal of producing skeletal replacement cells in culture and stimulating new skeleton to form after traumatic head injuries.
In the second year of this award, we have extended our initial findings that variant histones play an essential role in the generation of head skeletal cells. Skeletal cells usually arise from a cell population called the mesoderm. However, in the head a unique population of ectoderm cells, which normally forms derivatives such as neurons and skin, has an added ability to form skeletal cells. How these cranial neural crest cells acquire this extra potential remains unclear. Here we show that variant histones function within the ectoderm to give cranial neural crest cells skeletal-forming ability. In addition, we have used biochemical analysis in a human cell line to demonstrate how mutations in a particular H3.3 type of variant histone disrupt the association of histones with DNA. Furthermore, we have developed tools which allow us to analyze how variant histone changes throughout the genome endow neural crest ectoderm cells with mesoderm-like skeletal-forming potential.
Variant histones have been implicated in cell reprogramming, whereby a mature cell regains the ability to form many more cells that can repair a damaged tissue. As we find that variant histones are required for reprogramming of the ectoderm to form neural crest during early development, we believe that we can use this pathway to convert patient-specific cells to neural crest and skeletal cells that can be used for facial repair. To test this, we have developed a mouse model in which we can detect the ability of neural crest genes to convert mature cells to neural crest and skeletal fates. In parallel, we have developed a model of jaw regeneration in zebrafish that will allow us to test whether adult cells within the animal can be reprogrammed to repair the skeleton in response to injury. During this period, the generation of transgenic tools has allowed us to begin to address which cell types can give rise to new skeleton in response to injury. In the coming years, we hope that our work in model systems will lead to therapies for head skeletal injuries on two fronts: the generation of large amount of head skeletal precursors from patient-specific cells and the induction of increased regenerative ability of cells within the patient.